Method for the Production of Thin Dense Ceramic Layers

ABSTRACT

The invention relates to a method for the production of a thin dense ceramic layer on a substrate by means of atmospheric plasma spraying, whereby the following steps are carried out: a) the substrate is pre-heated to a temperature corresponding to at least a quarter of the melting point of the ceramic for application in Kelvin, b) a ceramic powder or a ceramic powder mixture with d 50 -values of less than 50 gm is used as spray adjunct, c) particle speeds at incidence on the substrate of more than 200 m/s are set, d) particle temperatures are set such that on incidence on the substrate surface the particles have a temperature at least 5% above the melting point of the ceramic for application in Kelvin, e) the amount of the spray adjunct and passage speed of the plasma burner are set such that on a single pass of the substrate a layer thickness of less than 100 ?m is achieved, f) a thin and also gas-tight layer is generated on the substrate with a single pass of the substrate which has a leakage rate of less than 10 −1  mbar L/(cm 2  s).

The invention relates to a method for producing ceramic layers, in particular ceramic layers with a thickness less than 100 μm and of gas-tight design.

PRIOR ART

Spraying techniques, in particular atmospheric plasma spraying, have proven to be very suitable for producing thin layers on a substrate.

In atmospheric plasma spraying (APS), spray additives in the form of particles or suspensions are applied by means of a plasma jet to the surface of a substrate to be coated. A plasma is a hot gas in which neutral particles dissociate and ionize due to high temperature. Thus, compared to the gas, charged particles such as electrons and ions are also present in a plasma.

To produce a plasma, an electric arc is generated between a cathode and an anode by means of high-frequency ignition in a plasma burner. At an appropriately selected gas feed, a concentrated plasma jet having a high heat content is formed that flows from the nozzle of the plasma burner at high speed. The temperatures in the hottest part of the plasma cone reach higher than 20,000 K. After the powder or suspension is introduced, there is a heat and force transfer to the powder particles that causes them to melt and accelerate. Depending on the parameters selected, the powder particles strike the substrate at a predetermined speed and temperature.

The correct setting of the spray parameters is, crucial for the quality and efficiency of the APS-generated layer. Process parameters to be set include in particular the flow rate and composition of the plasma gas and the powder carrier gas, the current, voltage, quantity of powder, particle speed and temperature, and substrate temperature, as well as the spraying distances and the relative velocities of the plasma burner and the substrate.

Ceramic layers produced by atmospheric plasma spraying generally have a variety of pore-like structures that may be categorized into two different types: cracks, and coarse, usually round, pores.

For cracks, a further distinction may be made between segmentation cracks and microcracks. The former run parallel to the coating direction through multiple spray lamellae, and sometimes even through the entire layer. The width of the crack opening is typically much greater than one micron. The microcracks are located between the lamellae (interlamellar) or in the lamellae (intralamellar), and have much smaller crack opening widths, generally less than one micron. The microcracks are interconnected like a network and consequently impart gas permeability to the layer. The gas permeability is likewise facilitated by the round-pores and segmentation cracks, in particular those that run through the entire layer.

For APS-produced layers according to the current prior art, these pore-like structures cannot be suppressed to the extent that the layers can consistently be referred to as gas-tight. In particular, production of gas-tight layers having leakage rates of less than 10⁻¹ mbar L/(cm² s) has not been possible heretofore without additional thermal aftertreatment.

OBJECT AND SOLUTION

The object of the invention is to provide a method for producing on a substrate thin and also gas-tight ceramic layers that in particular have a leakage rate of less than 10⁻¹ mbar L/(cm² s) without additional thermal aftertreatment.

The object of the invention is attained by a method comprising the totality of features according to the main claim. Advantageous embodiments of the method are stated in the claims that refer to the main claim.

SUMMARY OF THE INVENTION

The object of the invention is achieved by use of an atmospheric plasma spraying method in which a number of specialized parameters are adjusted while the method is being carried out. Through the combination of these parameters a thin ceramic layer, in particular having a thickness less than 100 μm, is deposited on a substrate, and the layer advantageously is gas-tight and has a leakage rate of less than 10⁻¹ mbar L/(cm² s).

Listed below are the parameters necessary for the atmospheric plasma spraying (APS) that result in the above-mentioned effect.

1. Production of the layers in one coating pass (only one layer). In other words, the plasma burner passes over the substrate only once during the coating operation. Performing only one pass reduces the tendency toward crack formation between the various spray layers. This is in contrast to the conventional technique in which multiple passes are typically done.

2. Setting the layer thickness to be deposited to a maximum of approximately 100 μm. This avoids the formation of segmentation cracks.

3. Preheating the substrate to a temperature that is at least 25% of the melting temperature, in Kelvin, of the ceramic used. This step improves the adhesion between the individual spray lamellae, partly by remelting of the already deposited particles.

4. Setting high incident particle speeds on the substrate at greater than 200 m/s, in particular greater than 250 m/s, by appropriate selection of process parameters. In this manner thin spray lamellae are produced that have a lesser tendency toward microcrack formation.

5. Setting high particle temperatures upon incidence on the substrate, with values at least 5%, preferably 10%, greater than the melting temperature by appropriate selection of parameters. This promotes remelting and formation of a composite; i.e. suppresses microcrack formation.

The above-listed process parameters may sometimes be achieved by virtue of the geometry of the plasma burner with respect to the substrate surface. Thus, the parameter settings under items 4 and 5 may generally be advantageously achieved by spraying distances that are not too large; i.e. typically less than 150 mm.

In addition, selection of the spray additive in the form of fine but flowable particles having d₅₀ values less than 50 μm, advantageously even less than 30 μm, facilitates setting of a high density in the layer to be deposited.

The speed of the robotic unit and the powder feed rate are selected so that a single pass produces a suitably dense layer having a layer thickness of less than 100 μm. Favorable robotic unit speeds are between 50 and 500 mm/s.

In particular ceramic materials having a melting point, for example zirconium oxide, as well as stabilizer additives such as perovskites, pyrochlores, aluminates, aluminum oxide, spinels, boron carbides, and titanium carbides, among others, have proven to be suitable materials for the above-referenced method.

The method according to the invention may be easily applied to the production of various layers, in particular for dense electrolytic layers for high-temperature fuel cells, membranes for gas separation technologies, and for oxidation- or corrosion-proof layers.

SPECIFIC DESCRIPTION

The subject matter of the invention is described in greater detail below with reference to one figure and two illustrated embodiments, without limiting the subject matter of the invention thereto.

A) Electrolytic Layers for High-Temperature Fuel Cells

Porous substrates provided with an anode were preheated to temperatures of approximately 500° C., using a plasma burner. Triplex II or F4 burners by Sulzer Metco, for example, may be used as the plasma burner. The power and the process gas flows were selected high enough to produce high process-gas speeds and temperatures. Used as powder was a melted, crushed, fully yttrium-stabilized zirconium oxide (YSZ) having a d₅₀ value of 20 μm. The incident particle speeds on the substrate were greater than 300 m/s, and the temperature was greater than 3000° C. The spraying distance was 90 mm. The substrate temperature during the coating was approximately 800° C. The speed of the robotic unit and the powder feed rate were selected such that one pass produced a dense YSZ layer approximately 90 μm thick. The robotic unit speed was set at 150 mm/s.

The YSZ layer thus produced generally had a leakage rate of less than 10⁻² mbar L/(cm² S).

The figure shows the layer structure of the above-referenced illustrated embodiment having a porous substrate, an intermediate layer thereon, and a dense YSZ electrolytic layer thereon that was applied by the APS method according to the invention.

B) Corrosion-Proof Layers for Fiber Composites

Fiber composites-made of carbon fiber composite (CFC) materials were provided with a mullite layer having cracks. This layer was coated with an additional gas-tight La₂Hf₂O₇ layer according to the invention as described herein in order to prevent attack by corrosive or oxidizing gases on the substrate and the mullite layer. Alternatively, a ceramic interlayer could be inserted to suppress the reactions between mullite and La₂Hf₂O₇.

Spray-dried powder having a d₅₀ of approximately 30 μm was used to produce the dense La₂Hf₂O₇ layer. The substrate was preheated to 400° C. using the burner (Triplex II, F4, or a higher-power burner variant). The particle speed was approximately 210 m/s at temperatures of approximately 2900° C. The layer produced was approximately 35 μm thick. 

1. A method for producing a thin dense ceramic layer on a substrate by atmospheric plasma spraying, comprising the following steps: preheating the substrate to a temperature corresponding to at least one-fourth of the melting temperature, in Kelvin, of the ceramic to be applied, using a ceramic powder or ceramic powder mixture having d₅₀ values less than 50 μm as spray additive, setting incident particle speeds on the substrate at greater than 200 m/s, setting particle temperatures such that upon incidence on the substrate surface the particles have a temperature that is at least 5% greater than the melting temperature; in Kelvin, of the ceramic to be applied, setting the quantity of the spray additive and the travel speed of the plasma burner such that a layer thickness of less than 100 μm is achieved by a single pass over the substrate, producing a thin and also gas-tight layer having a leakage rate of less than 10⁻¹ mbar L/(cm² s).
 2. The method according to preceding claim 1, further comprising the step of using a ceramic powder or ceramic powder mixture having d₅₀ values less than 30 μm as spray additive.
 3. The method according to claim 1, further comprising the step of introducing the powder introduced into the plasma flame in the form of a suspension.
 4. The method according to claim 1, further comprising the step of setting incident particle speeds on the substrate at greater than 250 m/s.
 5. The method according to claim 1, further comprising the step of setting particle temperatures such that upon incidence on the substrate surface the particles have a temperature that is at least 10% greater than the melting temperature, in Kelvin, of the ceramic to be applied. 